i used to think the yellow had to be on to operate shunt button 
but on Brit European trailers it works without the lines on
which is good cos you need to tip out the front sometimes
Thanks to all for the advice i kind of knew i was right. so just to cheer you up today i dropped a trailer applied the parking brake only to see the mad shunter hook up and the stop as he dragged the wheels it was quiet funny 
.
but you all are right. if i leave it in a safe mode anything that happens due to someone else is thier fault, lesson learnt 
Short answer
If you drop the red line the trailer parks on air, (as if you are sat there with you foot on the brake),
however if there is a leak in the footbrake side of the system and the air leaks away over time the brakes come back off and the trailer is sat there unbraked, this is what happens when you back under a trailer and it moves backwards rather than staying still
If you pull the park valve it exhausts the park side of the brake chambers and the trailer is now parked on springs, and in thoery should not go anywhere until the valve is pushed back in again
hitch:
i used to think the yellow had to be on to operate shunt button
About 5 or 6 years ago I came across a batch of trailers (new) where you had to have the Yellow line connected, and (I think) the Aux electrical, before they would move on the ‘shunt’ button.
I think it was a ‘fad’ by an individual manufacturer that didn’t last long.
Stevie, I emailed you a ‘Guide’ to coupling.